Marconi 703 Mastergram Fault - request for advice please. - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 10th Mar 2008, 12:11 am

Hi Peter,
The Marconi 703 employs two MSP4 pentodes in the sync separator stage. Each timebase has it's own sync valve.
The variable resistor VR2 setting is very critical, it adjust the cathode voltage of both sync valves.
The positive going video from the TRF unit will drive the MSP4s into conduction and because of the critical setting of VR2 the valves do not conduct on the sync pulses. The sync pulses present at the anodes are positive.
The resistor R2 supplies the screen grids of both valves, another resistor R32 (5000ohms) is connected between the screen grids and cathodes, thus created a constant voltage source for the screen grids.
Before removing the timebase unit again check the setting of VR2. Turn the contrast control fully clockwise and set the sensitivity control for a good contrasted picture, then adjust VR2 for best line and frame hold.

If this action fails then the timebase will have to come out again.

DFWB.

DoctorWho · 10th Mar 2008, 2:13 pm

Thanks for this advice David, thi sounds very helpful.

Sorry to sound a bit dim, but which control is VR2?

Once I know that then I'll follow the instructions you've given above. It may be that fitting the new resistors will require some adjustment as the old ones were so out of tollerance.

Thanks again,

Peter.

FERNSEH · 10th Mar 2008, 2:41 pm

Hi Peter,

In the Marconi 702 the sync control is situated above the two MSP4 sync separator valves. I guess it will be the same in the 703 timebase unit.

Regards,
David.

DoctorWho · 10th Mar 2008, 2:56 pm

Thanks David, now you some to mention it I have seen a variable resistor on the chassis in that area, it uses the same type of shaft as the other controls, but obviously has no knob on it. I did wonder what it was for. Now that I know I'll follow your guidelines and, hopefully, that may resolve the problem.

Thanks for this help, I'll ket you know if this sorts it out.

All the best,

Peter.

DoctorWho · 10th Mar 2008, 8:01 pm

Everything seems fine now, the set powers up and is stable right away. I'm not holding my breath, but, so far so good.

Many thanks for the help,

Peter.

10th Mar 2008, 12:11 am	#61
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Marconi 703 Mastergram Fault - request for advice please. Hi Peter, The Marconi 703 employs two MSP4 pentodes in the sync separator stage. Each timebase has it's own sync valve. The variable resistor VR2 setting is very critical, it adjust the cathode voltage of both sync valves. The positive going video from the TRF unit will drive the MSP4s into conduction and because of the critical setting of VR2 the valves do not conduct on the sync pulses. The sync pulses present at the anodes are positive. The resistor R2 supplies the screen grids of both valves, another resistor R32 (5000ohms) is connected between the screen grids and cathodes, thus created a constant voltage source for the screen grids. Before removing the timebase unit again check the setting of VR2. Turn the contrast control fully clockwise and set the sensitivity control for a good contrasted picture, then adjust VR2 for best line and frame hold. If this action fails then the timebase will have to come out again. DFWB.

10th Mar 2008, 2:13 pm	#62
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi 703 Mastergram Fault - request for advice please. Thanks for this advice David, thi sounds very helpful. Sorry to sound a bit dim, but which control is VR2? Once I know that then I'll follow the instructions you've given above. It may be that fitting the new resistors will require some adjustment as the old ones were so out of tollerance. Thanks again, Peter.

10th Mar 2008, 2:41 pm	#63
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Marconi 703 Mastergram Fault - request for advice please. Hi Peter, In the Marconi 702 the sync control is situated above the two MSP4 sync separator valves. I guess it will be the same in the 703 timebase unit. Regards, David.

10th Mar 2008, 2:56 pm	#64
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi 703 Mastergram Fault - request for advice please. Thanks David, now you some to mention it I have seen a variable resistor on the chassis in that area, it uses the same type of shaft as the other controls, but obviously has no knob on it. I did wonder what it was for. Now that I know I'll follow your guidelines and, hopefully, that may resolve the problem. Thanks for this help, I'll ket you know if this sorts it out. All the best, Peter.

10th Mar 2008, 8:01 pm	#65
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi 703 Mastergram Fault - request for advice please. Everything seems fine now, the set powers up and is stable right away. I'm not holding my breath, but, so far so good. Many thanks for the help, Peter.